Colon cancer is the second leading cause of death from cancer in the United States today whereas cancer of the small intestine is a relatively rare event. Intestinal tumors develop from a series of somatic mutations subsequent to an initiating event such as DNA-alkylation or oxidation. Our primary hypothesis is that the small intestine (SI) and large intestine (LI) respond differently to these two important forms of DNA damage (alkylation and oxidation), which is a key reason for the difference in tumor incidence at these two sites. Our secondary hypothesis is that SI cells are protected from tumor induction because they produce less reactive oxygen species (ROS) and less oxidative damage to DNA and LI cells. In specific aim # 1 we will inject rats with the DNA alkylating agent azoxymethane and measure in vivo DNA damage, repair and apoptosis in SI and LI over the 48 h period post injection. DNA damage and repair are measured by quantitative immunohistochemistry of O6-methylguanine adducts and its repair enzyme; and apoptosis is detected by the TUNEL assay. In specific aim #2 we will determine in vivo response to the DNA oxidizing agent dextran sodium sulfate in rat SI and LI within the first 48 h after removal of the oxidizing agent. DNA damage is measured by quantitative immunohistochemistry of 8oxodG adducts; activity of the repair enzyme specific for 8oxodG by an endonuclease assay; and apoptosis by the TUNEL assay. In specific aim #3 we will determine steady state levels of oxidative DNA damage and ROS generation in SI and LI in young and old rats and whether or not ROS generation in SI and LI is due to differences in mitochondrial electron transport. DNA damage will be assessed by the FLARE assay and ROS production by oxidation of the vital dye 2',7'-dichlorofluorescein in cells incubated with and without electron transport inhibitors.